The present invention relates to CDMA networks and to integrating the provision of stream-based services (eg., voice) and packet-based services.
In wireless systems, there exists a need to support a wide variety of multimedia services, such as voice, data, video, facsimile, and interactive services, simultaneously for many different users. The users, in turn, have differing requirements. For example, voice or other real time calls are examples of stream based services which require dedicated communication channels, as lengthy delays in transmission are not acceptable. However, certain data calls, for example electronic mail or other text messages, do not require dedicated communication channels as variable or lengthy delays in transmission can be accommodated.
In a conventional IS-95 CDMA (code division multiple access) system, packet data services are provided through traffic channels, which were originally designed for voice traffic. A packet data call request and the responses to the request are communicated through the paging and access channels. Then, a traffic channel is assigned to the packet data call in a circuit-switched mode. This method causes significant transmission overhead when the packet data traffic is bursty. A further drawback is that ATM cells cannot be transmitted directly in this system.
Accordingly, a need exists for a novel system structure and method to more efficiently and more reliably accommodate various services, including variable rate packet data services.
One aspect of the invention provides a novel common packet data channel (CPDC) system structure and method to efficiently provide variable-rate packet data services in a CDMA system. This system uses CPDC channels to serve packet data calls for a plurality of packet data users within a cell or sector. CPDC channels are defined for forward link (base station-to-terminal) communications and reverse link (terminal-to-base station) communications. Preferably, on the forward link (FL), an ATM-type multiplexing scheme is employed on a CPDC channel, while a spread ALOHA-type random access scheme is used on the reverse link (RL). Advantageously, the overhead and delays due to call setup and channel assignment in a conventional circuit-switched system is reduced for packets in this system. Overhead of idle transmissions for inactive terminals is also reduced. The preferred embodiment allows ATM cells to be directly transmitted over the air interface.
One advantage of CDMA systems compared to other types of digital cellular systems is that several terminals can communicate using the same frequency simultaneously. It is well known for CDMA systems that several virtual channels can be used by having each communication unit code their transmissions with a distinct spreading code sequence. Furthermore, only a communication unit with the appropriate code can decode such a coded message. This provides security against eavesdropping and also provides a multiple access scheme. These aspects of CDMA are well known in the art. It is also known that several communication units can communicate using the same spreading code as long as they are offset in time. This is currently utilized in the access channel, which is shared by a plurality of terminals in a cell/sector. Thus if, for example, three terminals send requests on the access channel using the same spreading sequence, the first received request can be successfully decoded, even if they overlap, provided they are offset in time. Of course in conventional systems, the subsequent requests are discarded. However, we have taken advantage of this property of a CDMA system to allow for the transmission of a plurality of overlapping signals (but offset in time) in the reverse link. Thus, a plurality of terminals can send overlapping transmissions of packet data using the same spreading code sequence to the base station. A base station according to a preferred embodiment of the invention can decode a plurality of overlapping frame signals by utilizing a plurality of CPDC receivers which each decode a separate signal. Thus, for example, a base station can receive multiple packet messages from a plurality of terminals all using the same spreading code sequence as long as there is some time offset between the reception of the plurality of signals by the base station.
Another aspect of the invention provides a terminal (and method of using said terminal) for using said CPDC system.